gralloc: Perform function to return custom stride

The custom stride can be set by video for smooth streaming.
Return this in perform so that graphics can use it.

Bug: 10192531
Change-Id: I798fc0894b8d9662e93769a53a540cb61b527a44
hwc: Disable idle invalidation for command mode panels

Command mode panels have an internal framebuffer with which they
refresh, hence the idle invalidator isn't needed for them.

Change-Id: I6acf0cc1f8c520cbcf706f109a42e47304c534d3